I assume price is the whole fitting including detaching lever, routing and re-attaching, if not also shortening cable? £30 would sound spot on for that.
Also they may be breaking open a bleed kit to do it. Small amount of fluid, but it’s a cost.
That said, I’ve usually been able to do Reverb cable shortening without needing a bleed. Unscrew lever, hose cutter, re-attach. Just be careful not to drip anything and don’t press the lever. Fully expected to need to bleed, but was fine each time. Only one I’ve had issues with was on swapping a RH lever for a LH but I think the lever wasn’t compatible with the Reverb model I had. I tried bleeding it loads of times and could never get it right.
Not through the books, they’ll do it cheap to avoid tax type of thing? Hmm 😉